1. Field of the Invention:
This invention relates to the determination of physical or chemical properties of compositions by means of near-infrared absorption bands. More specifically, this invention relates to compositions that involve hydrocarbons and substituted hydrocarbons, along with oxygenates such as ethers and alcohols. Still more specifically, this invention relates to octane determinations of oxygenate-containing fuels and percents by volume or by weight of oxygenate content of such fuels.
Oxygenate content for purpose of this specification and claims includes the oxygen that would be contained in hydrocarbons. Examples of typical oxygen-containing hydrocarbons that are particularly useful for automotive fuels are: alcohols, such as ethanol, methanol, butanol, iso-butanol, t-butanol, and iso-amyl alcohol; and ethers, such as methyl t-butyl ether, ethyl t-butyl ether, methyl iso-amyl ether, and ethyl iso-amyl ether.
2. Description of the Prior Art
The prior art as shown in the following U.S. patents shows that it is possible to find near-infrared absorption bands which correlate with octane. Note in this regard, European Patent Application (Publication number 0285251) 88301646.1 of Lambert et al. of BP Oil International Limited, filed Feb. 25, 1988. Heat content of certain fuel compositions and number of methyl groups per molecule were also shown to be correlatable to certain near-infrared absorption bands.
"Use of Indirect Multi-Variable Calibration Equations for Quality Control of Agricultural Products by Near-Infrared Spectroscopy," G. Puchwein and A. Eibelhuber (Mikrochim. Acta) Wein), (1986; It 43-51) teach a computer program for calibrating an instrument. The program picks relevant frequencies for spectral data for part of a set of samples with which to calibrate an instrument and then validates with respect to the remaining samples not used to establish to calibration.
A near-infrared regression model for octane measurements in gasoline which contain MTBE was published in a book of proceedings by the American Chemical Society, Division of Fuel Chemistry, in Volume 35, Number 1, Pages 266-275 which was written by Steven M. Maggard and also presented by him at the 199th American Chemical Society National Meeting in Boston, Mass. on Apr. 22-27, 1990. Specifically disclosed in this paper and presentation is the use of methyne t-butyl combination bands, including overtones, in models to predict octanes of MTBE-containing gasoline compositions.
B. G. Osborne, in an article entitled, Calibration of Instrument for Near-Infrared Spectroscopy, 48 Spectroscopy, Vol. No. 4, pages 48-55), discloses in detail the steps for establishing simple correlations by multiple linear regression and also the use of derivatives of linear terms. Validation of calibration curves are discussed: specifically how particular wavelengths are chosen to establish the terms in multiple linear regression equations, along with the statistical significance of a particular calibration sample. This method can be used in this invention for the purpose of defining the calibration equations with which one can calculate oxygenate content or octane from absorption spectral data.
Still other methods for determining calibration equations are those discussed in Analytical Chemistry Journal, volume 60, pages 1193-1202 (1988) by David M. Haaland and Edward V. Thomas. The principles of inverse least squares, partial least squares, classical least squares, and principle component regression analysis are disclosed. All of these methods are useful in the instant invention.
Jeffrey J. Kelly and James B. Callis in Analytical Chem. 1990, volume 62, pages 1444-1451, in an article entitled, Nondestructive Analytical Procedure for Simultaneous Estimation of Major Classes of Hydrocarbon Constituents of Finished Gasolines, discloses how frequencies are selected and how the statistical significance of a particular set of frequencies are determined. Specifically stepwise multilinear regression was performed. Mathematical treatments of the data prior to multiple regression analysis, such as base-line drift correction, end-point smoothing, and first and second derivatives, were explored to determine best-fit linear-regression equation for each data set. The inter-correlations between the regression wavelengths were calculated, and the wavelengths that exhibited high correlations were eliminated in order to reduce information redundancy and over fitting. This selection of wavelengths did not take into account the degree of temperature dependence.
Jeffrey J. Kelly, C. H. Barlow, T. M. Jinguji, and J. B. Callis in an article entitled, Prediction Of Gasoline Octane Numbers from Near-Infrared Spectral Features in the Range 660-1215 nm, published in Analytical Chemistry, Volume 61, No. 4, Feb. 15, 1989, on page 313-319, disclose use of multi-variable analysis of spectra to predict ASTM motor-determined octane numbers. Independent multi-variable analysis using partial leased-squares (PLS) regression analysis is also disclosed. The key step in the setting up of the calibration equations appears to be that high correlations were eliminated in order to reduce information redundancy and over-fitting.
U.S. Pat. No. 4,800,279 to Hieftje et al. (Jan. 24, 1989) teaches use of near-infrared (NIR) to predict physical properties of certain hydrocarbon mixtures.
Currently, programs exist to find and optimize correlations between near infrared absorption spectra for a particular material, or their mathematical transforms, and physical and chemical properties of such a material. Specific properties that are known to be indirectly determinable from near infrared spectra include: octane; cetane; enthalpy or heat content; percent water in grain; a PIANO analysis; and the like. Currently, the method followed in the art as presently practiced and known, is to essentially look for spectral properties that correlate with some property of interest, such as a physical or chemical property. This is both reasonable and logical. This approach is described in "The Analyst", Volume 107, published October 1982, pages 1282-1285 in an article entitled, Determination of Ethanol in Gasoline Mixtures by Near Infrared Method, by Jane L. Wong and Bruno Jaselskis. The article found that 1580 nanometers was a particularly useful absorption frequency for determining the % ethanol content of gasohol. However, neither suggested nor disclosed is the fact that a finite difference derivative such as discussed in Example 1 when taken at the first and higher levels of derivative can result in a two fold decrease in error from temperature dependence when the value of the dependent variable calculated. Preferably, in the case of a second derivative of absorbance in the range of 1576 to 1596 for a calibration equation that predicts volume percent oxygenate content the segment preferably in integral multiples of 2 nanometer lengths is in the range of 4 to 30, and more preferably 15 to 25 and the gap is an integer in the range, of 0 to 5 and more, preferably 0.
Alternatively, one would expect that the most intense bands representative of the functional groups of interest; e.g., in the case of oxygen-containing gasoline components, a CO or OH characteristic vibrational frequency in the near infrared would be of most interest. Examples of such bands for OH are in the first overtone near 1410 nanometers, in the second overtone near 1000 nanometers, and for combinations bands near 2000 nanometers. These frequencies are most likely to give the highest correlation to percent by volume oxygenate content. These frequencies are mentioned by L. G. Weyer in "APPLIED SPECTROSCOPY REVIEWS", Volume 21 (Issues 1 and 2), pages 1-43, published 1985 in an article entitled, Near-Infrared of Organic Substances.
This invention discloses the surprising discovery that there are particularly useful bands for determining oxygenate content of gasoline blends, that includes ethers and/or alcohols, that are overlooked in the "Applied Spectroscopy Reviews". One must use the absorption frequency in the range of 1,300-1,350, and preferably 1,310 through 1,340 nanometers. Once one limits the contribution in the correlation equation to expressly include those in this frequency range, one finds that the correlation can lead to a substantially reduced temperature dependence.
This invention further discloses the discovery that there is a markedly decreased temperature dependence in the calculated dependent variable that corresponds to any chemical or physical property. For example, there is a reduced temperature dependency in calculated octane or cetane when one correlates a derivative of absorption at unexpected frequencies in the range 1568 to 1596 preferably 1576 to 1592 nanometers. This becomes particularly important in those cases in which the spectral properties for the components making up the mixture are markedly temperature dependent. Such compounds are often those that give rise to hydrogen bonding such as methanol, ethanol, and other alcohols. The temperature dependence of any near infrared absorption results from a complex array of interactions including dipole-dipole interactions; including hydrogen bonding; density effects; and refractive index changes. Though 1580 has been reported as useful by Wong and Jaselskis, there was no suggestion or disclosure that as much as a ten fold decrease in temperature dependence could be achieved by taking finite difference derivatives, such as for example, shown in Examples Ill and IV.
That different functional groups have different degrees of temperature dependence makes multi-component analysis by NIR especially complex and difficult. Dipolar moments interacting with other dipolar moments tend to be temperature dependent. The variation in dipolar moments of various functional groups give rise to different degrees of temperature dependence.
Prior art analysis of composition by means of NIR, such as a PIANO which stands for the group acronym for: paraffins, isoparaffins, aromatics, naphthenes, and olefins involve determinations based upon total amount of those molecules taken together that are respectively members of some one of these groups. They did not determine the specific volume percent of a particular specie within any one of the various groups. Previous determinations of PIANO by NIR were limited to determining the compositions based upon groups of molecules rather than individual species. U.S. patent application Ser. No. 506,391, filed Apr. 9, 1990, to Maggard (Attorney Docket No. 6362AUS) discloses PIANO-NIR and the usefulness of PIANO bands for predicting octanes. Of course, use of techniques such as gas phase or liquid phase chromatography were able to isolate specific components of each of these species. The reason, however, that in an NIR-based determination, one is forced to deal with molecules by groups results from the extensive overlap with spectra from various species within a particular mixture. For example the CH bands of isoparaffins, and normal paraffins have large regions of absorption overlap. Consequently, to distinguish the contributions from one specie of a mixture separate from that contribution of another is very difficult, if not impossible. Attempts to do this become extremely complicated mathematically because one would have to determine each component's contribution to a particular band, and one would have to have enough bands so as to be able to distinguish how much of each specie was contributing to the various bands. If one had as many as ten unknowns, one would need at least nine different bands if all of the components had absorption bands that overlapped one another. J. D. Winefordner has done work detailing such requirements for such mixtures.